Firing sol-gel alumina particles

ABSTRACT

Sol-gel alumina that is dried but unfired can be explosively comminuted by feeding the dried gel into a furnace held at temperatures above those at which vaporizable materials are eliminated from the particles of gel. At suitably elevated temperatures the firing is sufficient to form fully densified alpha alumina particles of a size suitable for direct use as abrasive grits.

BACKGROUND OF THE INVENTION

This invention relates to aluminous abrasive grits and particularly tosol-gel alumina abrasive materials with improved grinding performance.

Sol-gel alumina abrasives are conventionally produced by drying a sol orgel of an alpha alumina precursor, (which is usually but notessentially, boehmite), at about 125 to 200° C. to remove the watercomponent of the gel; breaking up the dried gel into particles of thedesired size for abrasive grits; perhaps calcining the particles,(generally at a temperature of from about 400-800° C.), to form anintermediate form of alumina; and then finally firing the calcinedpieces at a temperature sufficiently high to convert them from anintermediate form such as gamma alumina to the alpha alumina form.Simple sol-gel processes are described for example in U.S. Pat. Nos.4,314,827; 4,518,397; 4,881,951 and British Patent Application2,099,012. Sol-gel aluminas of this type tend to have crystal sizes ofup to 25 microns or more, though modifying additives such as silica,spinel formers such as magnesia, and other metal oxide additives such aszirconia, yttria, rare earth metal oxides, titania, transition metaloxides and the like have been used in minor amounts to reduce thecrystal size to about 1 to 10 microns and enhance certain physicalproperties.

In a particularly desirable form of sol-gel process, the alpha aluminaprecursor is "seeded" with a material having the same crystal structureas, and lattice parameters as close as possible to, those of alphaalumina itself. The "seed" is added in as finely divided form aspossible and is dispersed uniformly throughout the sol or gel. It can beadded ab initio or it can be formed in situ. The function of the seed isto cause the transformation to the alpha form to occur uniformlythroughout the precursor at a much lower temperature than is needed inthe absence of the seed. This process produces a crystalline structurein which the individual crystals of alpha alumina, (that is those areasof substantially the same crystallographic orientation separated fromadjacent crystals by high angle grain boundaries), are very uniform insize and are essentially all sub-micron in diameter for example fromabout 0.1 to about 0.5 micron in diameter. Suitable seeds include alphaalumina itself but also other compounds such as alpha ferric oxide,chromium suboxide, nickel titanate and a plurality of other compoundsthat have lattice parameters sufficiently similar to those of alphaalumina to be effective to cause the generation of alpha alumina from aprecursor at a temperature below that at which the conversion normallyoccurs in the absence of such seed. Examples of such seeded sol-gelprocesses are described in U.S. Pat. Nos. 4,623,364; 4,744,802;4,954,462; 4,964,883; 5,192,339; 5,215,551; 5,219,806 and many others.

The optional calcining of the dried sol-gel is often preferred so as tominimize the time needed at the elevated firing temperatures. This isbecause the firing operation performs the tasks of converting thetransitional alumina forms to the alpha form and the sintering of thealpha alumina to close up residual porosity and ensure that theparticles have adequate density and hardness to function well asabrasive grits. It is known that excessive time at sinteringtemperatures, which are generally between 1300 and 1400° C. for seededsol-gel materials and about 100° C. higher than that for unseededsol-gel aluminas, can lead to crystal growth. Since crystal growth isgenerally regarded as undesirable, it is considered appropriate to carryout the calcining separately and so minimize the time at such elevatedtemperatures. This procedure is followed in spite of the extra cost ofmaintaining two high temperature operations.

Since the drying operation is followed by a crushing and screeningoperation the grain is reduced to room temperature and the heat used todry the grain is given up to the surroundings. This is of course veryinefficient.

The crushing operation is performed after the drying because, at thispoint the material is relatively easily broken up. If it were left tillafter the firing operation, the material would be so hard that excessiveamounts of energy would be required. It is therefore common-sense tocrush at the pre-fired stage. In addition it is considered that firingwill be more efficient since the particles will more rapidly reach thefiring temperature in the furnace if they are small.

It has now been found possible to significantly reduce the energyconsumption involved in the production of alumina by a sol-gel process.This is achieved by a manipulation of the process in a manner that iscompletely contrary to the intuitive reasoning used in designingconventional processes. The novel process produces alpha aluminaparticles in a very desirable form that is fully densified and welladapted to use in abrasive applications. Moreover the system is flexibleenough to permit design of the abrasive grits obtained.

GENERAL DESCRIPTION OF THE INVENTION

The process of the invention comprises feeding a dried but not firedsol-gel alumina having a volatilizable content of at least 5% by weight,directly into a furnace held at a temperature above 400° C. andcontrolling the temperature and residence time to produce an explosivelycomminuted alumina. Under certain conditions when the temperature in thefurnace is high enough and the time therein is long enough, the sol-gelalumina can be converted directly to the alpha alumina form and sinteredto a density that is at least 95% of the theoretical density.

Sol-gel alumina generally dries to form lumps a few millimeters in sizeand this is basically dried boehmite, each molecule of which has anassociated molecule of water, with perhaps some residual water notcompletely removed in the drying. In addition advantageous modifierssuch as magnesia, yttria, rubidia, caesia, or rare earth or transitionmetal oxides are often added to the sol-gel in the form of their solublenitrates and these too will contribute volatilizable components, (suchas nitrogen oxides), to the dried gel. If an acid such as nitric oracetic acid has been used to peptize the sol-gel there may also beresidues of this acid in the dried sol-gel. Generally the dried gel hasan overall vaporizable content of from about 5 to about 50%, preferablyfrom about 10 to about 45%, and more preferably from about 20 to about40% by weight. Drying is usually conducted at a temperature below about200° C. and more usually at temperatures far lower. For this reason thedried gel contains substantial amounts of vaporizable material when itis charged into the furnace.

Violent dehydration by rapid exposure to high temperatures is disclosedin EP-A-0 176 476 and EP-A-0 518 106 but is applied to fine powders ofhydrargillite and is intended as an intermediate step in the productionof boehmite or a reactive shaped body.

While the invention is primarily directed towards the explosivecomminution of dried sol-gel materials, these materials can also includeother components that do not themselves comprise any volatilizablematerial. Thus it is possible to include in the sol-gel materialcomponents such as alpha or gamma alumina powder, silicon carbide (bothparticulate and whisker forms), zirconia, cubic boron nitride, and otherabrasive materials, providing the overall content of vaporizablematerial in the dried mixture remains above the 5% by weight material.

When the lumps of dried gel are placed in the furnace the vaporizablematerial in the lumps expands explosively causing them to fly apartleaving smaller particles which are highly suitable for use in grindingapplications. If the residence time in the furnace is sufficiently long,the smaller particles that remain are rapidly converted to the alphaphase and sinter very quickly to the essentially fully densified form.The violent nature of this process has led to it being describedfamiliarly as "explosive comminution" though in a preferred embodimentthe process goes beyond comminution and includes firing to the alphaphase and, in some cases sintering to essentially theoretical density.Where the temperature is somewhat lower, the amount of comminution maybe reduced somewhat and may chiefly result in the break-up of the largerpieces and the creation of weakness lines in the remaining pieces thatrender them easily broken up in a subsequent comminution operation. Thishowever is also regarded as explosive comminution.

Therefore "explosively comminuted" material is understood to be formedwhen dried sol-gel alumina particles are fed into the furnace and are atleast partially broken into smaller particles without the use of anyexternally imposed force.

When the furnace residence time is relatively short or the furnacetemperature is relatively low, the sintering process and even theconversion to the alpha phase may not be completed when the materialexits the furnace. In such event some or all of the particles may beporous to some degree and these relatively loosely consolidated largerparticles may be broken up by a light milling operation before beingsintered to a density in excess of 95% of theoretical in a separatefurnace or by a second passage through the rotary furnace. This issometimes preferred since a very intense explosive comminution can leadto the production of significant amounts of very fine particles that maybe less useful in some abrasive applications. The less severe explosivecomminution has the effect of making even apparently unbroken particlesvery easy to comminute in a subsequent operation. Alternatively andsometimes preferably, the fired material that has not been completelyexplosively comminuted, which often has a degree of porosity, may be atleast partially impregnated with a volatilizable liquid such as waterand passed once more through the rotary furnace to complete thecomminution process.

Adjustment of the firing conditions to produce a product that is porousas described above also affords the opportunity of. impregnating theporous material with solutions of modifying agents such as for examplean aqueous solution of a soluble salt of magnesium, yttriium, atransition element, rubidium, caesium or a rare earth metal. Onsintering these materials will usually generate the modifying oxide in avery effective form and simultaneously generate further amounts ofvolatilizable material that can be used to bring about explosivecomminution.

It is found that if the gel is extruded before being dried, it isadvantageous if the extrusion orifice is smaller rather than larger.Thus gel extruded through a 1.6 mm orifice explosively comminutes betterthan gel extruded through an orifice 6 mm in diameter. In addition roundextrudates are preferred over angular extrudates such as those obtainedby extrusion through rectangular orifices.

Abrasive grain prepared in the above way often has an unexpectedlybetter grinding performance than grain obtained by more conventionalprocesses. It is theorized that this may be because the comminutiontechnique does not impose physical strains on the material of the typethat could give rise to micro-defects in the abrasive grit structure.Regardless of theory this performance improvement is surprising andsignificant.

The invention therefore also comprises novel aluminous abrasive grains.It is found that the grains obtained by explosive comminution have aunique shape and size distribution and this also may contribute to theexcellent grinding performance referred to above. They are differentfrom molded or extruded grains which have a uniform cross-sectionalshape as a result of their production process. Instead they have theirregular shape, and specifically in the cross-section along the longestdimension, that characterizes comminuted grain.

In general abrasive grits made by a non-shaping process are obtained bycomminuting larger pieces of material. There are two basic conventionaltechniques for performing such comminution: impact crushing and rollcrushing. Impact crushing tends to give a more blocky shape where theindividual grits have L/D, or aspect ratios, (the ratio of the longestdimension, (L) to the greatest dimension perpendicular to the longestdimension, (D)), close to 1. Grits produced by roll crushing tend tohave weaker shapes and in practice this means an average aspect ratiothat is greater than 1. Of course there is a range of actual aspectratios in roll crushed grits but most are substantially less than 2.

The "grit size" is conventionally measured using a series of sieves withdiffering size apertures in the mesh. If a grit is characterized bythree mutually perpendicular dimensions, the controlling dimension indetermining the "grit size" is the second largest since this willdetermine the size of the smallest hole through which the grit can passwhen oriented along its longest dimension. If the grits according to theinvention are on the average somewhat longer than conventional grits,they will clearly have a larger average volume per grit and this isindeed found to be the case.

The abrasive alumina grits according to the invention arenon-symmetrical about the longitudinal dimension and, as produced andwithin any grit size fraction, comprise more than 25%, preferably atleast 30% and more preferably at least 50%, of grits with an aspectratio of at least 2:1.

Conventional roll crushed alumina abrasive grits have been found tohave, within any grit size fraction as produced, no more than 25% ofgrits, and usually from about 19-25%, with an aspect ratio of 2:1 ormore. This appears to be a function of the process rather than thespecific alumina of which the alumina grits are composed. Thus the gritsaccording to the invention are identifiably different from the grits ofthe prior art. This is most clearly shown by the greatly increasedgrinding performance of the grits according to the invention as isillustrated by the Examples below.

DRAWINGS

FIG. 1 shows a DTA trace of a seeded sol-gel alumina.

FIG. 2 shows a simplified elevation of an apparatus adapted for theimplementation of one embodiment of the process of the invention.

DETAILED DESCRIPTION OF THE INVENTION

From the drawing presented as FIG. 1, which is a Differential ThermalAnalysis trace following a seeded sol-gel as its temperature is raised,it will be seen that there is an endotherm at about 400° C. Thisindicates the loss of volatiles including water and acid and saltdecomposition products. It is this loss of volatiles that causes theexplosive comminution. Clearly the faster this loss occurs, the moreexplosive the decomposition will be. By about 600° C. the amount ofvolatiles to be removed has significantly diminished and conversion tothe anhydrous phases of alumina such as gamma alumina is complete. Athigher temperatures still, the conversion to the alpha phase begins.With seeded sol-gel materials, this occurs at about 1150° C. or evenlower. This is indicated by the peak in FIG. 1. In an unseeded sol-gel,the trace will be very similar except that the alpha conversion peakwill occur at a rather higher temperature, perhaps 1250° C. or so.

To practice the invention it is only necessary to heat at a temperatureat which the volatiles begin to be driven off. Clearly highertemperatures than the minimum favor a very rapid decomposition that hasthe maximum explosive effect. However if the heating is sufficientlyrapid even modest temperatures at the lower end of the above ranges canbe used effectively.

If temperatures at the lower end of the above ranges, (that is wherealpha alumina has still not been formed), are used, the explosivelycomminuted material must be subjected to a further firing operation tocomplete the conversion to the alpha phase and (if desired) sinter thematerial to essentially theoretical density, (generally taken to be inexcess of 95%). While this involves further expense, it does allow theuse of rotary furnace materials that are much more sturdy and far lessexpensive that the silicon carbide tubes that are standard for furnacesin which all operations are performed at the same time.

Before being subjected to explosive comminution the sol-gel alumina istypically dried at temperatures below about 200° C. and more preferablyat much lower temperatures such as from about 75 to about 175° C.

As has been indicated above, it is highly desirable to provide that thelarge particles of dried sol-gel material are heated as rapidly aspossible to achieve the maximum expansion and explosive comminution. Theapparatus illustrated in simplified elevation and partial cross-sectionin FIG. 2 is well suited to meet these requirements. Uncrushed driedparticles of a sol-gel alumina about 0.5 to about 1 cm in diameter arefed into the hopper, 1, from which they are fed through a vibratoryfeeder, 2, to a secondary feeder, 3. This secondary feeder dischargesparticles into an air eductor, 4, which in turn accelerates theparticles using a stream of compressed air entering through port, 5,which carries the particles through a conduit, 6, and into a rotaryfurnace, 7, having upper and lower ends, at a point, 8, adjacent the hotzone within the furnace. In use the particles explode when they enterthe hot zone and comminuted particles exit the lower end, 9, of thefurnace.

In an explosive comminution process the heating of the lumps of driedgel is preferably done rapidly to achieve the maximum explosive effect.While several furnace designs other than that illustrated in FIG. 2could be adapted to meet this requirement, a highly suitable furnace forcarrying out the process is a rotary furnace comprising a tube inclinedat an angle to the horizontal and rotating about its axis, said tubebeing heated by externally applied heat. The rotation of the tubeensures that the lumps or particles inside the tube are in constantmovement such that no one part of a lump or particle is heated bycontact with the tube to the exclusion of another part. The speed ofrotation and the angle of incline of the tube determine the residencetime inside the furnace. These parameters are preferably adjusted toensure that the evaporation of the vaporizable materials from inside thelumps happens rapidly rather than gradually. This is to enable theparticles formed after the explosive breakup of the lumps to spend themaximum time firing and densifying.

Other furnace designs can be used as desired including batch furnacesoptionally with fluidized beds and furnaces with microwave or inductionheating.

A rotary furnace for use with firing temperatures of the order of thoseneeded to sinter alumina conveniently has a silicon carbide tube. Thisis because of its ability to stand up to the physical demands of theprocess including the temperature variations along the length and thedifferent loads at different point along the tube length. Siliconcarbide is also able to withstand any acidic gases that might begenerated, for example as nitrate residues are eliminated. If however itis intended to carry out the explosive comminution and conversion to thealpha form at temperatures below those at which full sintering occurs,it is possible to use metal alloys capable of withstanding temperaturesof up to about 1200° C. such as "Inconel".

Using a rotary furnace, the process of the invention requires aresidence time in the hot zone of from about 1 second to about 30minutes and preferably from about 2 seconds to about 20 minutes. Toachieve such residence times the angle of elevation of the tube ispreferably from about 1° to about 60° and more preferably from about 3°to about 20° and the speed of rotation is preferably about 0.5 to about20 rpm and more preferably from about 1 to about 15 rpm.

When firing a seeded sol-gel alumina the firing temperature in the hotzone of a rotary furnace is usually from about 400° C. to about 1500° C.and more preferably from about 600° C. to about 1400° C. For an unseededsol-gel alumina the hot zone is preferably maintained at a temperatureof from about 400° C. to about 1650° C. and more preferably from about600° C. to about 1550° C.

The particles obtained by the explosive comminution process of theinvention tend to have pronounced aspect ratios, that is, they have onedimension that is substantially longer than any other. Such particlesare particularly useful in coated abrasive applications.

The process of the invention is applicable to all types of sol-gelparticle production particularly where these are intended for abrasiveapplications. The sol-gel can be seeded or unseeded, the only differencein the conditions used is that a higher sintering temperature isgenerally required when the sol-gel is unseeded. It may also be appliedto sintered aluminas where finely divided alpha alumina particles areformed into blocks with partial sintering before being impregnated witha liquid and then explosively comminuted to produce abrasive particles.

Because the process of the invention permits the elimination of thephysical comminution stage typical of the prior art, the dried gel canbe fed directly into the furnace from the drier. This saves considerabletime and energy costs.

DESCRIPTION OF PREFERRED EMBODIMENTS

The process of the present invention is now described with particularreference to the firing of a seeded sol-gel alumina in a rotary furnace.These examples are for the sake of illustration only and are intended toimply no essential limitations on the essential scope of the invention.

Example 1

A Ross mixer was charged with 74,657 gm of deionized water and a slurryof alpha alumina seeds having a BET surface area of about 120 m² /gmmade by adding 6,000 gm of a 6% slurry of the seeds in deionized waterto 10,000 gm of deionized water. Boehmite, ("Disperal" sold under thattrademark by Condea GmbH), in an amount of 36.00 kg was also added andthe mix was evacuated and agitated for 5 minutes. A solution of 1,671 gmof 70% nitric acid in 5,014 gm of deionized water was then added whilethe stirred mixture was maintained under vacuum and stirred for afurther 5 to 10 minutes. The vacuum was then released and the mixturewas gelled by passing the mixture through an in-line mixer-homogenizerwhile injecting into the mixture a solution of 1,671 gm of 70% nitricacid in 5,014 gm of deionized water.

The gel was dried and broken up into lumps of from about 0.25 cm to 1 cmin size and these lumps were fed into a furnace. The dried sol-gel lumpswere fed directly into a rotary furnace comprising silicon carbide tube213 cm in length and 15 cm in diameter, with a 50 cm hot zone maintainedat 1405° C. The tube was inclined at 6° to the horizontal and rotated atabout 18 rpm.

The lumps were explosively comminuted to a range of particle sizes fromwhich 50T (>300 and <355μ) sized grits were separated for physicaltesting. The time for the material fired to transit the rotary furnacewas about 1 to 2 minutes. The fired grits had a density in excess of 3.8gm/cc and comprised microcrystallites of alumina about 0.2 micron indiameter.

For the sake of comparison the same sol-gel formulation was dried in thesame way, roll-crushed to produce -24 mesh (<710μ) particles which werethen calcined at about 800° C. before being fired in a conventionalmanner in a conventional rotary furnace. The grits comprised the samesubmicron alumina crystallites as those explosively comminuted accordingto the invention.

The two samples were then made up into abrasive belts using exactly thesame amounts of grit, backing, maker and size coats. Each belt carried590 gm of grit per square meter of surface area and was 6.4 cm wide and152.4 cm long. The belts were run at 9,000 surface meters per minute andwere used to cut a 304 stainless steel bar for 4 minutes under a watercoolant at an applied force of 6.8 kg.

The belt made using the conventional grits cut 74 g during this periodwhile the belt made with the explosively crushed grits cut 94 g, or a27% improvement over the conventional belt.

Example 2

Dried lumps of seeded sol-gel alumina at room temperature with a size ofabout +24T (>710μ) were fed directly at a rate of about 2.25 to about4.5 kg/hour into the hot zone of a rotary furnace maintained at 1000° C.using an apparatus substantially as described in FIG. 2. The furnace wasthe same as was used in Example 1 except that the tube was rotated atabout 10 rpm and was inclined at about 7° to the horizontal. The gelparticles were explosively comminuted in the furnace and the grit sizedistribution was as described in Table 1 below.

                  TABLE 1                                                         ______________________________________                                        SIZE RANGE       AMOUNT IN RANGE                                              ______________________________________                                        +30 (>600 μ)  41%                                                          -30 +40 (<600 μ >425 μ)                                                                  31%                                                          -40 +50 (<425 μ >300 μ)                                                                  11%                                                          -50 +60 (<300 μ >250 μ)                                                                   3%                                                          -60 (<250 μ)   4%                                                          ______________________________________                                    

In a separate operation the above explosively comminuted material wasfurther sintered to a density greater than 3.8 gm/cc and the size rangeof the sintered material was as shown in Table 2 below.

In the case of both sets of grits the alumina was in the form ofsub-micron crystallites.

                  TABLE 2                                                         ______________________________________                                        SIZE RANGE       AMOUNT IN RANGE                                              ______________________________________                                        +30 (>600 μ)  22%                                                          -30 +40 (<600 μ >425 μ)                                                                  38%                                                          -40 +50 (<425 μ >300 μ)                                                                  23%                                                          -50 +60 (<300 μ >250 μ)                                                                   9%                                                          -60 (<250 μ)   8%                                                          ______________________________________                                    

Example 3

This Example illustrates the novel abrasive grits of the invention andtheir preparation.

A green seeded alumina gel was prepared as follows: Into a high solidsJaygo mixer equipped with two sigma blades and an extrusion screw wereplaced 148 kg of a boehmite available from Condea under the registeredtrademark "Disperal and 40 kg of deionized water. This mixture wasagitated for about 5 minutes with the screw running in reversedirection. An aqueous alpha alumina slurry was then added, (29 kg of a4% solids dispersion of alpha alumina with a BET surface area of greaterthan 110 m² /gm) and the mixing was continued for a further 8 minutes. Acharge of 30 kg of 22% nitric acid was then added and mixing wascontinued for a further 20 minutes. Finally the extrusion screw was runin the forward direction to extrude the resulting gel through a 6.3 mmextrusion die. The extruded gel was then dried to a water content ofabout -30-35% by weight. The dried extruded gel was then divided intotwo parts.

A first part was roll crushed, calcined at 600-800° C., and thensintered in a rotary kiln to greater than 97% of the theoreticaldensity. The sintered grits were then screened to separate a 50T (>300and <355μ) size and these grits were evaluated for their aspect ratioand their grinding performance. This represented conventional,roll-crushed, weak-shaped, sol-gel alumina abrasive grits.

The second portion was processed according to the technique described inExample 2 to a density of greater than 97% of theoretical except thatthe material to be explosively comminuted was first screened to +10 mesh(>2mm) to remove fines. A similar 50T (>300 and <355μ) size was screenedfrom the explosively comminuted product. This fraction was alsosubjected to aspect ratio analysis and evaluation of the grindingperformance.

Aspect Ratio Analysis

The grits to be analyzed were screened to -45+50 (<355 and >300μ) for a50T size or -30+35 (<600 and >500μ) for a 36T size.

The equipment used comprised a Dage MTIO PA-81 high resolution black andwhite camera fitted with a Nikon Micro Nikkor 55 mm macro lens andmounted on a Bencher M2 photo stand to capture the grit images. Gritswere scattered on black paper, (for white grains), and a photograph withseveral grits in the field of view was taken. Lighting was provided onlyby the ceiling fluorescent light and shadows or overlighting wasavoided.

The camera was mounted on the photo stand using the top hole in themounting bracket and the center hole in the camera back, with thevertical traveler on the photo stand locked at about the 44 cm position.The lens aperture was set at F-2.8. The system was calibrated by placinga metric ruler on the base of the photo stand, focusing the camera andsetting the desired length of the line, which in practice turned out tobe 10 mm or 10,000 microns.

The black paper with the abrasive grits thereon was moved to differentfields of view to analyze different grits.

The images were captured and analyzed using a Compix C Imaging1280/Simple 51 software system. One sharpening operation was done in theimage enhancing mode to further help lock in the grit edges duringdetection. A binary image was then generated and the image was edited toensure that two abrasive grits are not touching one another or toeliminate an obviously distorted image. A minimum size range for adetected grit was set to 200 square microns in order to reject anybackground noise in the image associated with the paper. This minimumssetting has not been found to exclude any grits tested thus far.

The measurements collected using the Simple 51 software included area,maximum length and maximum breadth for each grit in the field of view.The practice has been to measure these parameters on at least 200-250grits per sample. The collected data was then transferred to Microsoft'sExcel® (Release 5.0) software to determine averages, standard deviation,aspect ratios and associated cumulative data.

Evaluation of Grinding Performance

The abrasive grits were deposited electrostatically in a standardcoating weight on a cloth backing having a phenolic adhesive coat. Theadhesive was then cured. A phenolic resin size coat was then appliedover the abrasive grits and the size coat was cured. The coated abrasivematerial was converted into an endless belt with a length of 152.4 cmand a width of 6.35 cm. This belt was tested in a fixed force mode, at alinear speed of 914.4 surface meters/minute, using an aqueous coolant,by cutting a stainless steel 304 bar under a force of 6.8 kg. The totalsteel cut in a 20 minute period was determined at the end of the test.

All grits made had a microcrystalline structure consisting of alphaalumina crystallines from about 0.2 to about 0.4 microns in diameter asmeasured by the average intercept method.

Results of Evaluations

    ______________________________________                                               GRAINS/100              GRINDING PERF.                                 SAMPLE WITH L/D >/= 2.0                                                                           RELATIVE % GMS CUT/20 MINS.                               ______________________________________                                        EX. 3  54           207        284                                            COMP. 3                                                                              25           100        199                                            ______________________________________                                    

From the above it is clear that the explosively comminuted grits werequite superior in grinding performance and were quite different from theweak shaped grain of the comparative sample in having a much higherproportion of grains with a L/D ratio of greater than or equal to 2.0.

Example 4

This Example runs parallel to Example 3 except that a slightly differentsol-gel process was used. In all other respects the Examples were thesame.

The process used to produce both the comparative grits and the gritsaccording to the invention was as follows: A mixing tank was chargedwith 908 kg of water; 118 kg of a diluted alpha alumina seeds slurrycontaining 4% by weight of alpha alumina seeds with a surface area ofgreater than 120 m² /gm, (made by milling an 8% aqueous dispersion ofsubmicron alpha alumina in a Sweco mill using Diamonite low purityalumina media); and 41 kg of 21% nitric acid. The mixture was stirredusing a high speed disperser blade and evacuated to remove air bubbles.The pH was found to be about 4. This dispersion was then homogenized bypumping it through an in-line homogenizer along with 21% nitric acid fedin at 0.6 liters/minute. The resulting gel was dried, to about 30-35%water content.

The dried gel was then split into two parts and further processed andevaluated as described in Example 3. The results were as follows:

    ______________________________________                                               GRAINS/100              GRINDING PERF.                                 SAMPLE WITH L/D >/= 2.0                                                                           RELATIVE % GMS CUT/20 MINS.                               ______________________________________                                        EX. 4  36           171        281                                            COMP. 4                                                                              21           100        212                                            ______________________________________                                    

This data also demonstrates that also with a low solids content process,there is significant difference between grains made by a conventionalprocess and those made by the process of the invention.

Example 5

This Example is the same process as is described in Example 3 except forthe size of the grits that are evaluated. In place of 50T grits, a 36Tsize fraction was separated and evaluated. The results are as followed:

    ______________________________________                                               GRAINS/100              GRINDING PERF.                                 SAMPLE WITH L/D >/= 2.0                                                                           RELATIVE % GMS CUT/20 MINS.                               ______________________________________                                        EX. 5  27           142        259                                            COMP. 5                                                                              19           100        149                                            ______________________________________                                    

Thus even though the relative number of the longer grits is only 142% ofthe number obtained by roll crushing in the conventional fashion, thepositive effect on the grinding performance is still quite astonishing.

Example 6

This Example shows the variation in dimensions and weight in sevenmaterials graded to the same standard 45/50 size. Three were differentsamples of a seeded sol-gel alumina material that had each beenexplosively comminuted in the manner claimed in the invention. Theothers included three that had been made by a rolls crushing processfrom seeded sol-gel alumina materials similar to those from which thegrits comminuted according to the invention had been made. The finalsample was a commercial alumina abrasive grain available from the 3MCorporation under the trade name "321 Cubitron". This grain isunderstood to be made by an unseeded sol-gel alumina process in whichthe alumina is modified by minor amounts of yttria and rare earth metaloxides. The grits are believed to be made by a mechanical crushingoperation. They have crystal structure that comprises alumina crystalswith a diameter of from about 1 to 7 microns by the average interceptmethod.

The results are set forth in Table 3 below:

                  TABLE 3                                                         ______________________________________                                               AV.        AV.       AV.                                                      LENGTH     WIDTH     HEIGHT   AV. VOL.                                 SAMPLE MICRONS    MICRONS   MICRONS  CUB. MIC.                                ______________________________________                                        INV-1  754        400       179      3.95                                     INV-2  872        399       269      3.90                                     INV-3  673        424       254      3.19                                     ROLL-1 597        450       299      2.22                                     ROLL-2 615        414       282      2.92                                     ROLL-3 601        450       226      2.87                                     321    649        396       231      2.27                                     ______________________________________                                    

The measurement techniques were those described in Example 3 aboveexcept that the height measurement was performed using a white lightinterferometry technique. The data show that although the gritsize-determining dimension, (the width), differed across the range ofsamples by only about 54 microns because of the common grit size of allthe samples (45/50), and both the average heights and widths coveredoverlapping ranges, the average lengths and consequently the averageweights occupied clearly distinct ranges with the grits crushedaccording to the invention being longer and heavier than the prior artrolls crushed grits.

What is claimed is:
 1. Alumina abrasive grits that are non-symmetricalabout their longitudinal dimension and, as produced and within any gritsize fraction thereof, comprise more than 25% of grits with an aspectratio of at least 2:1 and have a density that is at least 95% of thetheoretical density.
 2. Alumina abrasive grits according to claim 1 inwhich the percentage of grits with an aspect ratio greater than 2:1 isgreater than 30%.
 3. Alumina abrasive grits according to claim 1comprised of sintered alumina crystals with sizes from 0.01 to 10microns.
 4. Alumina abrasive grits according to claim 3 in which thealumina crystallites are sub-micron in size.
 5. Alumina abrasive gritsaccording to claim 1 comprised of sintered alumina modified by theincorporation of up to 10% of one or more oxides selected from the groupconsisting of the oxides of magnesium, zirconium, rare earth metals,transition metals, rubidium, caesium and yttrium.